Catheter drive shaft clutch

ABSTRACT

Rotating element catheters employ clutch assemblies for preventing rotational energy from being transmitted from a motor drive unit to the catheter element under defined circumstances. The catheter assembly includes an elongate member in which there is disposed a rotatable catheter drive cable. To control the rotation of the catheter drive shaft, the clutch assembly is configured to allow the catheter drive shaft to rotate and to stop rotating. The clutch assembly includes a driven member and a driver member, which cooperate with each other, such that the driven and driver members are rotatably engaged with each other before an applied torque exceeds the critical magnitude, rotatably unengaged with each other after the applied torque exceeds the critical magnitude, and rotatably re-engaged when the applied torque falls substantially below the critical magnitude. This clutching function can be actuated by any of a variety of physical forces, e.g., frictionally or magnetically.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/548,690filed on Apr. 13, 2000, which issued as U.S. Pat. No. 6,475,224 on Nov.5, 2002, which is related to application Ser. No. 09/548,692 filed onApr. 13, 2000, which issued as U.S. Pat. No. 6,517,528 on Feb. 11, 2003,which is related to application Ser. No. 09/548,564 filed on Apr. 13,2000, which issued as U.S. Pat. No. 6,454,717 on Sep. 24, 2002, all ofwhich are expressly incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to the manufacture of catheters, andmore particularly to the manufacture of catheters having rotatableoperative elements.

BACKGROUND

Currently, there exist rotating element catheters, which can be used byphysicians to provide a diagnostic or therapeutic effect within the bodytissue of a patient, e.g., ultrasonic imaging or artherectomy. A typicalrotating element catheter includes a flexible drive cable that extendsthe length of the catheter body, terminating proximally in a motor driveunit. An operative element, e.g., an ultrasonic transducer orartherectomy blade, is distally mounted to the drive cable. Operation ofthe drive unit rotates the drive cable, which, in turn, rotates theoperative element at high speeds to produce the desired diagnostic ortherapeutic effect. Due to the nature of placing indiscriminatelyrotating elements inside a patient, there is always a risk that therotating element could inadvertently damage tissue if the catheter isdefective or mishandled.

For example, some ultrasonic imaging catheters can providetwo-dimensional 360° images along the length of a blood vessel byrotating an ultrasonic transducer at high speeds, while linearly movingthe ultrasonic transducer in the distal direction relative to thecatheter member. If the distal end of the catheter member is kinked, orotherwise formed into a tight curve, there exists the possibility,however so slight, that the rotating ultrasonic transducer couldperforate through the catheter member and damage the surrounding tissue.This is caused, in part, by the fact that the drive unit is designed tomaintain the speed of the transducer at a set level, accordinglyincreasing or decreasing the torque that is applied to the drive cable.In doing so, the drive unit does not discriminate between normalfrictional loads, i.e., frictional loads caused by normal frictionbetween the drive cable and catheter member, and abnormal frictionloads, i.e., frictional loads caused by an abnormal circumstance, e.g.,the boring of the transducer through the wall of the catheter member.

As a precaution, these types of ultrasonic imaging catheters aredesigned, such that the drive shaft fails if the torque required torotate the ultrasonic transducer becomes too great. This designcontemplates providing a circumferential space between the drive cableand the catheter member along a portion of the catheter, allowing thedrive cable to wind or ball up within the space when the torque appliedto the drive cable exceeds a critical magnitude. Presumably, such anexcess in force will occur if the rotating ultrasonic transducer beginsto perforate the catheter member, resulting in a failed drive cable, andpreventing the ultrasonic transducer from further boring through thecatheter member.

Typically, however, the drive shaft fails, not because the ultrasonictransducer is boring through the catheter member, but rather because thedrive cable is subjected to excessive frictional forces. Such forces areoften a result of having to route the catheter through the tortuousvasculature of a patient, forcing the drive cable to rotate through manycurves. Any mishandling of the catheter while operating the motor driveunit, e.g., overtightening the touhy-borst valve through which thecatheter is introduced into the patient, exacerbates this situation.Because the drive unit is designed to maintain the rotation of theultrasonic transducer at a uniform speed, the motor drive unit increasesthe torque that is applied to the drive cable to compensate for anyincrease in frictional force, thereby risking failure of the drivecable. In fact, of all the failed ultrasonic imaging catheters returnedto the assignee of this application, approximately seventy percent failas a result of this phenomenon.

There thus remains a need to prevent premature failure of a drive cablewithin a catheter, while minimizing the potential risk of inadvertentlydamaging tissue by the rotating operative element distally mounted onthe drive cable.

SUMMARY OF THE INVENTION

The present inventions are broadly directed to rotating elementcatheters and catheter assemblies that employ clutch assemblies toprevent rotational energy from being transmitted from a motor drive unitto the catheter element under defined circumstances.

In accordance with a first aspect of the present inventions, a catheterassembly includes an elongate member in which there is disposed arotatable catheter drive shaft, e.g., a flexible drive cable. Thecatheter drive shaft may have an operative element, e.g., an ultrasonictransducer or an artherectomy blade, distally mounted thereon forproviding diagnostic or therapeutic functions to the physician. In thecase of ultrasonic imaging, the elongate member can take the form of atelescoping guide sheath slidably disposed about an imaging core (i.e.,the catheter drive shaft and ultrasonic transducer) to provide thephysician with two-dimensional 360° ultrasonic images of surroundingbody tissue. To control the rotation of the catheter drive shaft, thecatheter assembly is mechanically coupled to the proximal end of thecatheter drive shaft, and is configured such that the catheter driveshaft is operated in a drive mode (i.e., it is allowed to rotate) and ina release mode (i.e., it is prevented from rotating). The clutchassembly may be located entirely within the catheter, e.g., in aproximal hub configured to interface with a motor drive unit, oralternatively, a portion or the entirety thereof may be locatedelsewhere within the catheter assembly, e.g., in the motor drive unititself.

In accordance with a second aspect of the present inventions, the clutchassembly is automatic and operates the catheter drive shaft in the drivemode before the torque applied to the proximal end of the catheter driveshaft exceeds a critical magnitude, and operates the catheter driveshaft in the release mode after the torque applied to the proximal endof the catheter drive shaft exceeds the critical magnitude. The criticalmagnitude can be adjusted or selected by the manufacturer of thecatheter assembly by “tuning” the clutch assembly.

In accordance with a third aspect of the present inventions, the clutchassembly can be formed from a driven member, which is rotatably coupled(either directly or indirectly) to the proximal end of the catheterdrive shaft, and a driver member, which cooperates with the drivenmember, such that the driven and driver members are rotatably engagedwith each other before the applied torque exceeds the criticalmagnitude, and rotatably unengaged with each other after the appliedtorque exceeds the critical magnitude. This clutching function of thedriven and driver members can be actuated by any of a variety ofphysical forces, e.g., frictionally or magnetically.

Other and further objects, features, aspects, and advantages of thepresent invention will become better understood with the followingdetailed description of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate both the design and utility of preferredembodiments of the present invention, in which:

FIG. 1 is a schematic view of an ultrasonic imaging system constructedin accordance with the present inventions;

FIG. 2 is a longitudinal section of a first preferred embodiment of anautomatic clutch assembly employed in the system of FIG. 1;

FIG. 3 is a side view of the clutch assembly of FIG. 2;

FIG. 4 is a side view of a second preferred embodiment of an automaticclutch assembly employed in the imaging system of FIG. 1;

FIG. 5 is a diagram showing the magnitude of a torque applied to acatheter drive shaft within the imaging system over a time period inresponse to a varying frictional load of the catheter drive shaft;

FIG. 6 is a side view of a third preferred embodiment of an automaticclutch assembly employed in the imaging system of FIG. 1;

FIG. 7 is a side view of a fourth preferred embodiment of an automaticclutch assembly employed in the imaging system of FIG. 1;

FIG. 8 is a partially cut-away side view of a fifth preferred embodimentof an automatic clutch assembly employed in the imaging system of FIG.1;

FIG. 9 is a partially cut-away side view of a sixth preferred embodimentof an automatic clutch assembly employed in the imaging system of FIG.1;

FIG. 10 is a cross-sectional view taken along the line 10—10 of FIG. 9;

FIG. 11 is a partially cut-away side view of a seventh preferredembodiment of an automatic clutch assembly employed in the imagingsystem of FIG. 1;

FIG. 12 is a cross-sectional view taken along the line 12—12 of FIG. 11;

FIG. 13 is a partially cut-away side view of an eighth preferredembodiment of an automatic clutch assembly employed in the imagingsystem of FIG. 1;

FIG. 14 is a cross-sectional view taken along the line 14—14 of FIG. 13;

FIG. 15 is a side view of a ninth preferred embodiment of an automaticclutch assembly employed in the imaging system of FIG. 1;

FIG. 16 is a side view of a tenth preferred embodiment of an automaticclutch assembly employed in the imaging system of FIG. 1;

FIG. 17 is a cross-sectional view taken along the line 17—17 of FIG. 16;

FIG. 18 is a side view of an eleventh preferred embodiment of anautomatic clutch assembly employed in the imaging system of FIG. 1;

FIG. 19 is a cross-sectional view taken along the line 19—19 of FIG. 18;

FIG. 20 is a side view of a twelfth preferred embodiment of an automaticclutch assembly employed in the imaging system of FIG. 1;

FIG. 21 is a cross-sectional view taken along the line 21—21 of FIG. 20;

FIG. 22 is a cross-sectional view taken along the line 22—22 of FIG. 20;

FIG. 23 is a side view of a thirteenth preferred embodiment of anautomatic clutch assembly employed in the imaging system of FIG. 1;

FIG. 24 is a cross-sectional view taken along the line 24—24 of FIG. 23;

FIG. 25 is a cross-sectional view taken along the line 25—25 of FIG. 23;

FIG. 26 is a side view of a fourteenth preferred embodiment of anautomatic clutch assembly employed in the imaging system of FIG. 1;

FIG. 27 is a cross-sectional view taken along the line 27—27 of FIG. 26;

FIG. 28 is a cross-sectional view taken along the line 28—28 of FIG. 26;

FIG. 29 is a side view of a fifteenth preferred embodiment of anautomatic clutch assembly employed in the imaging system of FIG. 1;

FIG. 30 is a cross-sectional view taken along the line 30—30 of FIG. 29;and

FIG. 31 is a cross-sectional view taken along the line 31—31 of FIG. 29.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an exemplary ultrasound imaging catheter system100, constructed in accordance with the present invention, is providedfor ultrasonically imaging a patient's internal body tissue 194, e.g.,the wall of an artery. The catheter system 100 generally includes aflexible ultrasonic imaging catheter 102, which houses an ultrasonicimaging core 108, a motor drive unit 104 (MDU) for providing a source ofrotational energy to the imaging core 108, and an ultrasonic signalprocessing unit 106 operatively connected to the imaging core 108 forproviding an ultrasonic image of the targeted tissue to a physician.

The catheter 102 includes an elongate telescoping catheter body 110,which facilitates the rotational and longitudinal translation of theimaging core 108. In particular, the catheter body 110 includes an outerguide sheath 112 with an imaging lumen 114. The imaging core 108 isdisposed within the imaging lumen 114, allowing the imaging core 108 tobe rotationally and longitudinally translated with respect to the guidesheath 112.

The imaging core 108 comprises a flexible catheter drive shaft 110,i.e., a drive cable, with an ultrasonic transducer 118 distally mountedthereon. As is well known in the art, the transducer 118 is composed ofa layer of piezoelectrical material, with acoustic matching and backinglayers suitably formed on the opposite sides thereof (not individuallyshown). The drive cable 116 is preferably designed, such that itpossesses a high torsional stiffness and a low bending stiffness. Forexample, the drive cable 116 can be made of two counterwound layers ofmultifilar coils that are fabricated using techniques disclosed inCrowley et al., U.S. Pat. No. 4,951,677, the disclosure of which isfully and expressly incorporated herein by reference. Thus, thetransducer 118 rotates about a longitudinal axis in response to theapplication of a torque on the proximal end of the drive cable 116. Theimaging core 108 further includes signal wires 114 (shown in FIG. 2),which are suitably connected to the transducer 118 by suitable means,e.g., welding. The signal wires 114 are routed through the drive cable116 from the transducer 118, extending out the proximal end of the drivecable 116.

The outer guide sheath 112 can be generally divided into three sections:an acoustic window 120, a main section 122, and an telescoping section124. The acoustic window 120 houses the transducer 118, and when filledwith a suitable imaging solution, allows ultrasonic energy U_(E) to betransmitted between the transducer 118 and the surrounding body tissue.The proximal end of the acoustic window 120 is suitably bonded to thedistal end of the main section 122, which extends almost the entirelength of the guide sheath 112. The main section 122 is characterized bya relatively stiff structure, which not only facilitates advancement ofthe catheter body 110 through the tortuous vasculature of the patient,but also facilitates advancement of the imaging core 108 through theimaging lumen 114. The distal end of the telescoping section 124 issuitably bonded to the proximal end of the main section 122, andincludes a semi-rigid tube 125 through which a smaller diametersemi-rigid tube 126 is slidably disposed. The semi-rigid tube 126extends proximally from the telescoping section 124 and serves toprovide rigidity to the drive cable 116 outside of the guide sheath 112.

In this regard, the semi-rigid tube 126 includes a lumen 128 throughwhich the proximal end of the drive cable 116 extends. Although thedrive cable 116 rotates relative to the semi-rigid tube 126, as will bedescribed in further detail below, the drive cable 116 and semi-rigidtube 126 are longitudinally affixed with respect to each other. Thus,relative translation of the semi-rigid tube 126 in the distal directionnecessarily translates the imaging core 108 in the distal direction withrespect to the guide sheath 112. Similarly, relative translation of thesemi-rigid tube 126 in the proximal direction necessarily translates theimaging core 108 in the proximal direction with respect to the guidesheath 112. To facilitate the telescoping action of the catheter 102,the telescoping section 124 includes an anchor housing 130 forconnection to a rigid pullback arm 190 of the MDU 104, as will bedescribed in further detail below.

The catheter 102 further includes a proximal hub 132, which mates with ahub 186 of the MDU 104. The catheter hub 132 provides the necessarymechanical interface between the imaging core 108 and the MDU 104, aswell as the electrical interface between the imaging core 108 and thesignal processing unit 106. In the illustrated embodiment, the catheterhub 132 is configured as a male adapter, with the MDU hub 186 beingconfigured as a female adapter.

Referring specifically to FIG. 2, the catheter hub 132 includes a rigidhousing 134 composed of a suitable material, e.g., plastic, and ismolded in a shape that facilitates firm seating of the catheter hub 132within the MDU hub 186. The housing 134 further includes a pair ofspring clamps (not shown), which interact with the MDU hub 186 toremovably affix the catheter hub 132 therein.

The proximal end of the housing 134 includes a transverse wall 136 fromwhich opposing distally and proximally extending cylindrical walls 138and 140 extend. The cylindrical walls 138 and 140 respectively includecavities 142 and 144, which are in communication with each other throughthe transverse wall 136. The semi-rigid tube 126 is permanently fixedwithin the distal cylindrical wall cavity 142 using adhesive 146. Inthis regard, the semi-rigid tube 126 is affixed to and extends throughthe adhesive 146, across the transverse wall 136, and into the proximalcylindrical wall cavity 144. A flexible rubber grommet 148 is suitablymounted to the distal end of the housing 134, around the distalcylindrical wall 138 and abutting the distal face of the transverse wall136. The grommet 148 receives and provides stress relief for the drivecable 116 and semi-rigid tube 126.

The catheter hub 132 further includes an automatic clutch assembly 200,which is firmly and rotatably seated within a cavity 152 of an innercylindrical wall 150 formed within the housing 134. The cylindrical wall150 is an axial alignment with the distal and proximal cylindrical walls138 and 140, and thus, the clutch assembly 200 is in axial alignmentwith the drive cable 116. The clutch assembly 200 is configured toadvantageously operate the drive cable 116 in either a drive mode or arelease mode. Specifically, when a torque T is applied to the proximalend of the drive cable 116, the clutch assembly 200 provides a means forpermitting rotation of the drive cable 116 before the applied torque Texceeds a critical magnitude (drive mode), and provides a means forpreventing rotation of the drive cable 116 after the applied torque Texceeds the critical magnitude (release mode).

To this end, the clutch assembly 200 comprises a driven member 202 and adriver member 204, which, as will be described in further detail below,interact with each other to provide the aforementioned clutchingfunction. The driven member 202 comprises a generally cylindrical rigidmember 208, which is composed of a suitable rigid material, e.g.,stainless steel. The cylindrical member 208 includes an elongate shaft210 with a proximally facing boss 212. The boss 212 and the shaft 210can be molded as an integral unit, or can alternatively be affixed toeach other using suitable means, e.g., welding.

The driven member 202 is rotatably coupled to the drive cable 118.Specifically, the driven member 202 is held in axial alignment with thedrive cable 118 by a bushing 154, which is composed of a suitably rigidbearing material, e.g., bronze. The bushing 154 is suitably bondedwithin the cavity 152 of the cylindrical wall 150, with the boss 212 ofthe driven member 202 being rotatably disposed with the bushing 154.Likewise, a seal 156 is suitably bonded within the cavity 144 of thecylindrical wall 140, with the shaft 210 being rotatably disposed withinthe seal 156. The driven member 202 is rotatably engaged with the drivecable 116 by suitably mounting the distal end of the shaft 210 to theproximal end of the drive cable 116, e.g., by welding. It is noted thata portion of the shaft 210 is hollow, which allows the signal wires 114from the drive cable 116 to extend therethrough.

The driver member 204 comprises a generally cylindrical rigid member214, which is molded from a suitably rigid material, e.g., plastic. Thecylindrical member 214 includes a proximally facing receptacle 216 witha cavity 218 formed therein for receiving the distal end of a rigidmotor drive shaft 184 from the MDU 104 (shown in FIG. 1), when thecatheter hub 132 is mated with the MDU hub 186. To facilitate proper andfirm engagement with the motor drive shaft 184, the receptacle 216 andmotor drive shaft 184 are keyed, such that the receptacle 216 rotatablyengages the motor drive shaft 184 when inserted into the cavity 218. Thedriver member 204 is held in longitudinal abeyance by a rigid arcuatemember 158, which is mounted through the housing 134 and engages anannular recess 160 formed in the cylindrical member 214.

The driver member 204 further includes a coil spring 206, whichintegrally rotates with and is affixed to the cylindrical member 214. Aswill be discussed in further detail below, the coil spring 206 interactswith the cylindrical rigid member 208 of the driven member 202 in amanner that actuates the clutching action between the driven member 202and the driver member 204.

The catheter hub 132 further includes an inductive coupler 162, which isfirmly seated within the housing 134 in an axial relationship with theclutch assembly 200. The inductive coupler 162 provides the means forinductively coupling the electrical energy from the signal wires 114,which rotate by virtue of their association with the rotating drivecable 116, and a stationary platform, i.e., the signal processing unit106. To this end, the inductive coupler 162 includes a disk-shapedmagnetic rotor 164 and a disk-shaped magnetic stator 166, which arelocated adjacent each other in a coaxial manner. The shaft 210 of thedriven member 202 extends entirely through the inductive coupler 162,where it is rotatably engaged with the rotor 164. Thus, the rotor 164 ofthe inductive coupler 162 integrally rotates with the driven member 202.The signal wires 114 extend from a transverse hole (not shown) made inthe shaft 210, and are suitably connected to the rotor 164. Lead-insignal wires 168 are mounted between the stator 166 and an electricaljack 170 mounted on the housing 134. In this manner, electrical signalscan be transmitted between the electrical jack 170 and the signal wires114 within the drive cable 116 when the imaging core 108 is rotating.

The catheter hub 132 further includes an infusion port 172 formed fromthe housing 134, which is in fluid communication with the cavity 144 ofthe distal cylindrical wall 140. Because the lumen 128 of the semi-rigidtube 126 (shown in FIG. 1) is in fluid communication with the cavity144, the infusion port 172 is in fluid communication with the imaginglumen 114 of the guide sheath 112. Thus, the acoustic window 120 can befilled with a suitable imaging fluid, e.g., a saline solution,introduced through the infusion port 172.

Referring back to FIG. 1, the MDU 104 provides the means forrotationally and longitudinally translating the imaging core 108 withrespect to the guide sheath 112. In particular, the MDU 104 comprises acasing 180 in which there is firmly affixed a motor 182 and theaforementioned motor drive shaft 184 (motor and shaft shown in phantom).As briefly discussed above, the MDU hub 186 mates with the proximalcatheter hub 132, with the distal end of the motor drive shaft 184 beingrotatably engaged with the driver member 204 of the clutch assembly 200.The casing 180 is mounted to a carriage 188 and do is in a slidingrelationship therewith. A drive train (not shown) is coupled between thecasing 180 and the motor 182, and is configured to longitudinallytranslate the casing 180 with respect to the carriage 188 in acontrolled manner when engaged with the motor 182.

Further details regarding the use of a single motor to actuate bothrotation of a drive shaft and longitudinal translation of a drive unitcasing are disclosed in U.S. Pat. No. 6,004,271, the disclosure of whichis fully and expressly incorporated herein by reference. Alternatively,separate and distinct motors can be used to respectively actuaterotation of the motor drive shaft 184 and longitudinal movement of thecasing 180. Further details regarding the use of two motors torespectively actuate rotation of a drive shaft and longitudinaltranslation of a drive unit casing are disclosed in U.S. Pat. No.6,013,030, the disclosure of which is fully and expressly incorporatedherein by reference.

The MDU 104 further includes a rigid pull back arm 190, one end of whichis mounted to the anchor housing 130 of the guide sheath 112, and theother end of which is mounted to the carriage 188. In this manner, whenthe MDU 104 is operated, the rotating imaging core 108 longitudinallytranslates in relation to the guide sheath 112, since the imaging core108 is longitudinally engaged with the casing 180 via the catheter hub132, and the guide sheath 112 is fixed in place by the pullback arm 190.

The MDU 104 includes feedback circuitry with an encoder (not shown),which senses the loss of rotational speed in the presence of anincreased friction force between the imaging core 108 and the catheterbody. In response, the feedback circuitry increases the currentdelivered to motor 182, maintaining the motor drive shaft 184 at the setspeed. This increased current translates to an increased torque Tapplied to the proximal end of the drive cable 116.

The signal processing unit 106 generally comprises a controller, datainterpretation unit, monitor, keyboard, etc. (not individually shown).The signal processing unit 106 is electrically coupled to the transducer118 of the imaging core 108 through the MDU 104. Specifically, apower/data cable 192 transmits input/output data between the MDU 104 andsignal processing unit 106, while providing DC electrical power to theMDU 104. Upon mating of the catheter hub 132 with the MDU hub 186, theMDU 104 is, in turn, electrically coupled to the imaging core 108 viasignal wires 114 connected to the electrical jack 170 (shown in FIG. 2).

During operation, the signal processing unit 106 transmits electricalsignals to the transducer 118 via the afore-described electrical path.In response, the transducer 118 is electrically excited, emittingultrasonic energy U_(E) through the acoustic window 120 into thesurrounding body tissue. The ultrasonic energy U_(E) is reflected fromthe surrounding body tissue, back through the acoustic window 120, andinto the transducer 118. The ultrasonic excited transducer 118, in turn,emits electrical signals, which are transmitted back to the signalprocessing unit 106 via the electrical path. By virtue of the fact thatthe transducer 118 is being simultaneously rotated and longitudinallytranslated during this process, the received electrical signalsrepresent a multitude of 360°0 data slices, which are constructed by thesignal processing unit 106 into a two-dimensional image of the bodytissue.

As stated above, the MDU 104 attempts to maintain the speed of the motordrive shaft 184 at a set speed, by increasing or decreasing the torqueapplied to the motor drive shaft 184 in response to a variablefrictional load. The clutch assembly 200, however, provides a check onthe MDU 104. In the presence of normal frictional loads, the clutchassembly 200 automatically engages the motor drive shaft 184 with thedrive cable 116, in which case, the drive cable 116 rotates with themotor drive shaft 184 (drive mode). In the presence of abnormalfrictional loads, however, the clutch assembly 200 automaticallydisengages the motor drive shaft 184 from the drive cable 116, in whichcase the drive cable 116 does not rotate with the motor drive shaft 184(release mode).

Referring to FIG. 3, the motor drive shaft 184 (shown partially inphantom) is shown applying the torque T to the proximal end of the drivecable 116 (via the clutch assembly 200) in a clockwise direction. Asnoted above, the current magnitude of the applied torque T at any giventime depends on the frictional load. Taking the current magnitude of theapplied torque T into account, the clutch assembly 200 allows the drivecable 116 to be alternately operated between the drive mode and therelease mode. To this end, the driven member 202 and the driver member204 are conditionally affixed to each other. That is, the driven member202 is rotatably engaged with the driver member 204 before the currentmagnitude of the applied torque T exceeds the critical magnitude, and isrotatably unengaged with the driver member 204 after the currentmagnitude of the applied torque T exceeds the critical magnitude.

In particular, the coil spring 206 is affixed to the cylindrical member214 of the driver member 204 by bending the proximal end of the coilspring 206 into engagement with a hole 220 formed in the cylindricalmember 214. Alternatively, the coil spring 206 can be affixed to thecylindrical member 214 by bending the distal end of the coil spring 206into engagement with the hole 220, as shown in the automatic clutchassembly 300 depicted in FIG. 4.

Referring back to FIG. 3, the coil spring 206 provides the means foreffecting the aforementioned clutching action. Specifically, the body ofthe coil spring 206, which, in the illustrated embodiment, isrepresented by seven and one-half coils 222, is interference fitted overthe boss 212, such that a frictionally engaging relationship is formedtherebetween. In this regard, the normal inner diameter (the innerdiameter in the absence of an external force) of the coil spring 206 isslightly less than the outer diameter of the boss 212. Preferably, theouter surface of the driven member 202 is polished to a substantiallyuniform diameter to provide a substantially uniform contact between thecoil spring 206 and boss 212.

The coil spring 206 is preferably wound in a direction, such that ittends to “unwind” in the presence of the applied torque T. That is, theinterference fit between the coil spring 206 and the boss 212 decreasesas the applied torque T increases. Thus, if the proximal end of the coilspring 206 is affixed to the cylindrical member 214 of the driver member204 (as shown in FIG. 2), the coil spring 206 is wound in thecounterclockwise direction from the proximal end. In contrast, if thedistal end of the coil spring 206 is affixed to the cylindrical member214 of the driver member 204 (as shown in FIG. 3), the coil spring 206is wound in the clockwise direction from the proximal end.

By way of nonlimiting example, the outer and inner diameters of the coilspring 206 can be 0.160 and 0.124 inches, with the diameter of the wirebeing 0.018 inches. Assuming an exemplary interference fit between thecoil spring 206 and driven member 202 of between 0.001 and 0.002 inches(in the absence of an applied torque), the outer diameter of the boss212 is preferably between 0.122 and 0.123 inches.

The operation of the clutch assembly 200 will now be described. FIG. 5specifically depicts the magnitude of the applied torque T (solid line)and the magnitude of a representative frictional load variance in thedrive cable 116 (dashed line) over time. FIG. 5 also indicates theparticular mode in which the drive cable 116 is operated, assuming thatthe drive cable 116 is initially operated in the drive mode. Note thatthe magnitude of the applied torque T tracks the magnitude of thefrictional load, which results from the tendency of the MDU 104 tomaintain the motor drive shaft 184 at a uniform speed. The lag betweenthe magnitude of the applied torque T and the magnitude of thefrictional load represents the time taken for the MDU 104 to adjust themagnitude of the applied torque T in response to the change in themagnitude of the frictional load.

As can be seen from FIG. 5, as long as the frictional load remainsnormal, the current magnitude of the applied torque T remains below thecritical magnitude. Thus, operation of the drive cable 116 is maintainedin the drive mode. Specifically, as long as the critical magnitude isnot exceeded, the current magnitude of the applied torque T does notovercome the frictional force generated by the interference fit betweenthe coil spring 206 and the boss 212 (in spite of the reducedinterference fit due to the “unwinding” of the coil spring 206 in thepresence of the applied torque T). Thus, the driven member 202 remainsrotatably engaged with the driver member 204. As a result, the drivecable 116 is rotatably coupled to, and integrally rotates with, themotor drive shaft 184. That is, the drive cable 116 is operated in thedrive mode.

As can be seen from FIG. 5, once the frictional load becomes abnormal,the drive cable 116 is operated in the release mode. Specifically, oncethe critical magnitude is exceeded, the current magnitude of the appliedtorque T overcomes the frictional force generated by the interferencefit between the coil spring 206 and the boss 212 (facilitated by thedecrease in the interference fit due to the “winding” of the coil spring206 in the presence of the applied torque T). Thus, the driven member202 becomes rotatably unengaged with the driver member 204. As a result,the drive cable 116 is rotatably uncoupled from, and does not integrallyrotate with, the motor drive shaft 184. That is, the drive cable 116 isoperated in the release mode.

As can be seen from FIG. 5, once the drive cable 116 is operated in therelease mode, the current magnitude of the applied torque T drops to alevel well below the critical magnitude. At this point, the currentmagnitude of the applied torque T tracks the magnitude of the frictionalforce between the rotatably unengaged coil spring 206 and boss 212,which generally remains uniform. The substantial drop in the currentmagnitude of the applied torque T is due to the frictional changes inthe clutch assembly 200. Specifically, the transition from a rotatablyengaged relationship to a rotatably unengaged relationship.(i.e.,transition from drive mode to release mode) is determined by africtional force between the coil spring 206 and boss 212 that is basedupon a stationary coefficient of friction. Once this transition is made,the frictional force between the coil spring 206 and the boss 212 isbased upon a dynamic coefficient of friction, which, as is well known,is less than the stationary coefficient of friction. The reducedfrictional force translates to a reduced applied torque needed tomaintain the motor drive shaft 184 at a uniform set speed.

As long as the MDU 104 maintains rotation of the motor drive shaft 184,once the drive cable 116 is operated in the release mode, operation ofthe drive cable 116 does not return to the drive mode until thefrictional load of the drive cable 116 drops below the frictional forcebetween the unengaged coil spring 206 and boss 212. It can thus be saidthat the clutch assembly 200 has a built-in hysteresis, ensuring thatthe drive cable 116 will not be operated in the drive mode until thefrictional load is well within the normal range, e.g., by retracting thecatheter or loosening the touhy-borst valve. Once this occurs, operationof the drive cable 116 returns to the drive mode, and the currentmagnitude of the applied torque T again tracks the magnitude of thefrictional load. It should be noted that the imaging core 108 can berepeatedly cycled between the drive mode and release mode withoutwearing out the clutch assembly 200 due to the intrinsic ability of thecoil spring 206 to consistently return to its normal diameter.

FIG. 6 depicts an alternative embodiment of an automatic clutch assembly400, which is constructed in accordance with the present inventions.Like the clutch assembly 200 described above, the clutch assembly 400includes a driven member 402 and a driver member 404 that areconditionally affixed to each other, wherein the clutching function ofthe clutch assembly 400 is frictionally actuated by the coil spring 206.The clutch assembly 400 differs from the clutch assembly 200, however,in that the driven member 402, rather than the driver member 404,includes the coil spring 206.

Specifically, the driven member 402 is similar to the above-describeddriven member 202 (see FIG. 3), with the exception that it includes thecoil spring 206, which is affixed to the cylindrical member 208 bybending the distal end of the coil spring 206 into engagement with theboss 212 by suitable means, e.g., welding. Alternatively, the coilspring 206 can be affixed to the cylindrical member 208 by bending theproximal end of the coil spring 206 into engagement with the boss 212,as shown in the automatic clutch assembly 500 depicted in FIG. 7.

Referring back to FIG. 6, the driver member 404 includes a generallycylindrical rigid member 414, which is constructed similarly to theabove-described cylindrical member 214 (see FIG. 3), with the exceptionthat the cylindrical member 414 includes a distally facing transitionalshaft 420. The body of the coil spring 206 is interference fit about thetransitional shaft 420 in the same manner as that described above withrespect to the coil spring 206 and boss 212 (see FIG. 3). Again, thecoil spring 206 is preferably wound in a direction, such that it tendsto “unwind” in the presence of the applied torque T. Thus, if the distalend of the coil spring 206 is affixed to the boss 212 (as shown in FIG.6), the coil spring 206 is wound in the clockwise direction from theproximal end. In contrast, if the proximal end of the coil spring 206 isaffixed to the boss 212 (as shown in FIG. 7), the coil spring 206 iswound in the counterclockwise direction from the proximal end.

The operation of the clutch assembly 400 is identical to that of theclutch assembly 200, with the exception that the coil spring 206frictionally interacts with the transitional shaft 420 of the drivermember 404, rather than the boss 212 of the driven member 202.

FIG. 8 depicts another alternative embodiment of an automatic clutchassembly 600, which is constructed in accordance with the presentinvention. Like the clutch assembly 200 described above, the clutchassembly 600 includes a driven member 602 and a driver member 604 thatare conditionally affixed to each other. The clutch assembly 600 differsfrom the clutch assembly 200, however, in that the driver member 604resides in the MDU 104, rather than in the catheter hub 132 (shown inFIG. 1).

Specifically, the driver member 604 comprises the motor drive shaft 184itself. The driven member 602 includes the above-described cylindricalmember 208 (see FIG. 3), as well as a generally cylindrical rigid member614, which is molded from a suitably rigid material, e.g., plastic. Thecylindrical member 614 includes a distally facing receptacle 620 with acavity 622 formed therein, wherein the boss 212 (shown partially inphantom) of the cylindrical member 208 is mounted by suitable means,e.g., bonding. Like the above-described cylindrical member 214 (see FIG.3), the cylindrical member 614 further includes a proximally facingreceptacle 616 with a cavity 618 formed therein for receiving the distalend of a rigid motor drive shaft 184 from the MDU 104 (shown in FIG. 2),when the catheter hub 132 is mated with the MDU hub 186. The receptacle616 and motor drive shaft 184, however, are not keyed, such that themotor drive shaft 184 freely rotates with the cavity 618 absentrestraint.

The driven member 602 further includes the coil spring 206, which isseated within an annular recess 626 formed within the cavity 618, withthe distal end of the coil spring 206 being suitably mounted to thereceptacle 616 distally adjacent the cavity 618. The diameter of theannular recess 626 is slightly greater than the normal outer diameter ofthe coil spring 206, whereby expansion of the coil spring 206 isallowed, i.e., the coil spring 206 is allowed to “unwind.” The normalinner diameter of the coil spring 206 is slightly smaller than the outerdiameter of the distal end of the motor drive shaft 184, such that thecoil spring 206 can be interference fitted over the distal end of themotor drive shaft 184. Again, the coil spring 206 is preferably wound ina direction, such that it tends to “unwind” in the presence of theapplied torque T. In the illustrated embodiment, the coil spring 206 iswound in the clockwise direction from the proximal end. As can be seen,the cavity 618 within the receptacle 616 tapers to a diameter equal tothe diameter of the distal end of the motor drive shaft 184. Thus, whenthe distal end of the motor drive shaft 184 is inserted into thereceptacle 616, it is guided into an interference fitted with the coilspring 206.

The operation of the clutch assembly 600 is identical to that of theclutch assembly 200, with the exception that the coil spring 206frictionally interacts with the motor drive shaft 184 of the drivermember 604, rather than the boss 212 of the driven member 202.

FIGS. 9 and 10 depict another alternative embodiment of an automaticclutch assembly 700, which is constructed in accordance with the presentinventions. Like the clutch assembly 200 described above, the clutchassembly 700 allows the drive cable 116 to be alternately operatedbetween the drive mode and the release mode, as dictated by themagnitude of the applied torque T. To this end, the clutch assembly 700includes a driven member 702 and a driver member 704, which areconditionally affixed to each other. That is, the driven member 702 isrotatably engaged with the driver member 704 before the currentmagnitude of the applied torque T exceeds the critical magnitude, and isrotatably disengaged from the driver member 704 after the currentmagnitude of the applied torque T exceeds the critical magnitude. Unlikethe clutch assembly 200, however, the clutch assembly 700 utilizes awatch spring 706, rather than the coil spring 206, to effect thefrictional clutching action.

Specifically, the driver member 704 comprises a generally cylindricalrigid member 714, which is molded from a suitably rigid material, e.g.,plastic. The cylindrical member 714 includes a proximally facingreceptacle 716 with a cavity 718 formed therein for receiving the distalend of a rigid motor drive shaft 184 from the MDU 104, when the catheterhub 132 is mated with the MDU hub 186 (shown in FIG. 2). To facilitateproper and firm engagement with the motor drive shaft 184, thereceptacle 716 and motor drive shaft 184 are keyed, such that thereceptacle 716 rotatably engages the motor drive shaft 184 when insertedinto the cavity 718. The cylindrical member 714 further includes adistally facing transitional shaft 720 and the watch spring 706, whichis wound around the transitional shaft 720, with one end of the watchspring 706 being suitably bonded to the transitional shaft 720 (bestshown in FIG. 10).

The driven member 702 comprises a generally cylindrical rigid member708, which is composed of a suitable rigid material, e.g., stainlesssteel. The cylindrical member 708 includes an elongate shaft 710 with aproximally facing receptacle 712 having a cavity 713 formed therein. Thereceptacle 712 and the shaft 710 can be molded as an integral unit, orcan alternatively be affixed to each other using suitable means, e.g.,welding.

The watch spring 706 provides the means for effecting the aforementionedclutching action. Specifically, the watch spring 706 is interferencefitted within the cavity 713, such that a frictionally engagingrelationship is formed between the watch spring 706 and the receptacle712. In this regard, the normal outer diameter (the outer diameter inthe absence of an external force) of the watch spring 706 is greaterthan the inner diameter of the cavity 713. Preferably, the cavity 713 ispolished to a substantially uniform diameter to provide a substantiallyuniform contact between the watch spring 706 and the receptacle 712. Thewatch spring 706 is preferably wound in a direction, such that it tendsto “wind” in the presence of the applied torque T. That is, theinterference fit between the watch spring 706 and the receptacle 712decreases as the applied torque T increases. In the illustratedembodiment, the watch spring 706 is wound in the counterclockwisedirection from the inside.

The operation of the clutch assembly 700 is similar to that of theclutch assembly 200 described with respect to FIG. 5. Specifically, aslong as the critical magnitude is not exceeded, the current magnitude ofthe applied torque T does not overcome the frictional force generated bythe interference fit between the watch spring 706 and the receptacle 712(in spite of the reduced interference fit due to the “winding” of thewatch spring 706 in the presence of the applied torque T). Thus, thedriven member 702 remains rotatably engaged with the driver member 704.As a result, the drive cable 116 is rotatably coupled to, and integrallyrotates with, the motor drive shaft 184. That is, the drive cable 116 isoperated in the drive mode.

Once the critical magnitude is exceeded, the current magnitude of theapplied torque T overcomes the frictional force generated by theinterference fit between the watch spring 706 and the receptacle 712(facilitated by the decrease in the interference fit due to the“winding” of the watch spring 706 in the presence of the applied torqueT). Thus, the driven member 702 becomes rotatably unengaged with thedriver member 704. As a result, the drive cable 116 is rotatablyuncoupled from, and does not integrally rotate with, the motor driveshaft 184. That is, the drive cable 116 is operated in the release mode.

FIGS. 11 and 12 depict another alternative embodiment of an automaticclutch assembly 800, which is constructed in accordance with the presentinventions. Like the clutch assembly 700 described above, the clutchassembly 800 includes a driven member 802 and a driver member 804 thatare conditionally affixed to each other, wherein the clutching functionof the clutch assembly 800 is frictionally actuated by the watch spring706. The clutch assembly 800 differs from the clutch assembly 700,however, in that the driven member 802, rather than the driver member804, includes the watch spring 706.

Specifically, the driven member 802 includes a generally cylindricalrigid member 808, which is constructed similarly to the above-describedcylindrical member 708 (see FIG. 9), with the exception that thecylindrical member 808 does not include a receptacle 712. Thus, thecylindrical member 808 is formed solely by an elongate shaft 810. Thedriven member 802 further includes the watch spring 706, which is woundaround the proximal end of the shaft 810, with one end of the watchspring 706 being suitably bonded to the shaft 810 (best shown in FIG.12).

The driver member 804 includes a generally cylindrical rigid member 814,which is constructed similarly to the above-described cylindrical member714 (see FIG. 9), with the exception that the cylindrical member 814includes a distally facing receptacle 820 having a cavity 822 formedtherein, rather than the transitional shaft 720. The watch spring 706 isinterference fitted within the cavity 822 in the same manner as thatdescribed above with respect to the watch spring 706 and the cavity 714of the receptacle 712 (see FIG. 9). Again, the watch spring 706 ispreferably wound in a direction, such that it tends to “wind” in thepresence of the applied torque T. In the illustrated embodiment, thewatch spring 706 is wound in the clockwise direction from the inside.

The operation of the clutch assembly 800 is identical to that of theclutch assembly 700, with the exception that the watch spring 706frictionally interacts with the receptacle 820 of the driver member 804,rather than the receptacle 712 of the driven member 702.

FIGS. 13 and 14 depict another alternative embodiment of an automaticclutch assembly 900, which is constructed in accordance with the presentinvention. Like the clutch assembly 700 described above, the clutchassembly 900 includes a driven member 902 and a driver member 904 thatare conditionally affixed to each other. The clutch assembly 900 differsfrom the clutch assembly 200, however, in that the driver member 904resides in the MDU 104, rather than in the catheter hub 132 (shown inFIG. 1).

Specifically, the driver member 904 comprises the motor drive shaft 184itself. The driver member 904 further includes the watch spring 706,which is wound around the distal end of the drive shaft 184, with oneend of the watch spring 706 being suitably bonded to the motor driveshaft 184 (best shown in FIG. 14).

The driven member 902 includes a generally cylindrical rigid member 908,which is constructed similarly to the above-described cylindrical member708 (see FIG. 9), with the exception that the cylindrical member 908includes a proximally facing boss 912, rather than the receptacle 712.The driven member 902 further includes a generally cylindrical rigidmember 914, which is molded from a suitably rigid material, e.g.,plastic. The cylindrical member 914 includes a distally facingreceptacle 920 with a cavity 922 formed therein, wherein the boss 912(shown partially in phantom in FIG. 13) of the cylindrical member 908 ismounted by suitable means, e.g., bonding. Like the above-describedcylindrical member 714 (see FIG. 9), the cylindrical member 914 furtherincludes a proximally facing receptacle 916 with a cavity 918 formedtherein for receiving the distal end of a rigid motor drive shaft 184from the MDU 104, when the catheter hub 132 is mated with the MDU hub186 (shown in FIG. 2).

The watch spring 706 is interference fitted within the cavity 918 in thesame manner as that described above with respect to the watch spring 706and the cavity 713 of the receptacle 712 (see FIG. 9). Again, the watchspring 706 is preferably wound in a direction, such that it tends to“wind” in the presence of the applied torque T. In the illustratedembodiment, the watch spring 706 is wound in the counterclockwisedirection from the inside.

The operation of the clutch assembly 900 is identical to that of theclutch assembly 700, with the exception that the watch spring 706frictionally interacts with the receptacle 916 of the driven member 902,rather than the receptacle 712 of the driven member 702.

FIG. 15 depicts another alternative embodiment of an automatic clutchassembly 1000, which is constructed in accordance with the presentinventions. Like the clutch assemblies 200 and 700 described above, theclutch assembly 1000 allows the drive cable 116 to be alternatelyoperated between the drive mode and the release mode, as dictated by themagnitude of the applied torque T. To this end, the clutch assembly 1000includes a driven member 1002 and a driver member 1004, which areconditionally affixed to each other. That is, the driven member 1002 isrotatably engaged with the driver member 1004 before the currentmagnitude of the applied torque T exceeds the critical magnitude, and isrotatably disengaged from the driver member 1004 after the currentmagnitude of the applied torque T exceeds the critical magnitude. Unlikethe clutch assemblies 200 and 700, however, the clutch assembly 1000utilizes a compliant member 1006, rather than a spring, to effect thefrictional clutching action.

Specifically, the driver member 1004 comprises a generally cylindricalrigid member 1014, which is molded from a suitably rigid material, e.g.,plastic. The cylindrical member 1014 includes a proximally facingreceptacle 1016 with a cavity 1018 formed therein for receiving thedistal end of a rigid motor drive shaft 184 from the MDU 104, when thecatheter hub 132 is mated with the MDU hub 186 (shown in FIG. 2). Tofacilitate proper and firm engagement with the motor drive shaft 184,the receptacle 1016 and motor drive shaft 184 are keyed, such that thereceptacle 1016 rotatably engages the motor drive shaft 184 wheninserted into the cavity 1018. The cylindrical member 1014 furtherincludes a distally facing transitional shaft 1020 and the complianttube 1006, which is composed of a suitably compliant material, e.g.,rubber or silicone. The proximal end of the compliant tube 1006 isdisposed over and suitably bonded to the transitional shaft 1020.

The driven member 1002 comprises a generally cylindrical rigid member1008, which is composed of a suitable rigid material, e.g., stainlesssteel. The cylindrical member 1008 includes an elongate shaft 1010 witha proximally facing boss 1012. The boss 1012 and the shaft 1010 can bemolded as an integral unit, or can alternatively be affixed to eachother using suitable means, e.g., welding.

The compliant tube 1016 provides the means for effecting theaforementioned clutching action. Specifically, the distal end of thecompliant tube 1006 is interference fitted over the boss 1012, such thata frictionally engaging relationship is formed therebetween. In thisregard, the normal outer diameter (the outer diameter in the absence ofan external force) of the compliant tube 1006 is less than the outerdiameter of the boss 1012. Preferably, the boss 1012 is polished to asubstantially uniform diameter to provide a substantially uniformcontact between the compliant tube 1006 and the boss 1012. Because theinner diameters of the proximal and distal ends of the compliant tube1006 are the same, the outer diameter of the boss 1012 is preferablyequal to the outer diameter of the transitional shaft 1020.

Although the compliant tube 1006 in the illustrated embodiment isconditionally affixed to the boss 1012, the compliant tube 1006 canalternatively be conditionally affixed to the transitional shaft 1020.That is, the distal end of the compliant tube 1006 can be disposed overand suitably bonded to the boss 1012, and the proximal end of thecompliant tube 1006 can be interference fitted over the boss 1012, suchthat a frictionally engaging relationship is formed therebetween.

The operation of the clutch assembly 1000 is similar to that of theclutch assembly 200 described with respect to FIG. 5. Specifically, aslong as the critical magnitude is not exceeded, the current magnitude ofthe applied torque T does not overcome the frictional force generated bythe interference fit between the compliant tube 1006 and the boss 1012(or the compliant tube 1006 and the transitional shaft 1020 if thecompliant tube 1006 is conditionally affixed to the transitional shaft).Thus, the driven member 1002 remains rotatably engaged with the drivermember 1004. As a result, the drive cable 116 is rotatably coupled to,and integrally rotates with, the motor drive shaft 184. That is, thedrive cable 116 is operated in the drive mode.

Once the critical magnitude is exceeded, the current magnitude of theapplied torque T overcomes the frictional force generated by theinterference fit between the compliant tube 1006 and the boss 1012 (orthe compliant tube 1006 and the transitional shaft 1020 if the complianttube 1006 is conditionally affixed to the transitional shaft). Thus, thedriven member 1002 becomes rotatably unengaged with the driver member1004. As a result, the drive cable 116 is rotatably uncoupled from, anddoes not integrally rotate with, the motor drive shaft 184. That is, thedrive cable 116 is operated in the release mode.

FIGS. 16 and 17 depict another alternative embodiment of an automaticclutch assembly 1100, which is constructed in accordance with thepresent inventions. Like the clutch assemblies 200, 700, and 1000described above, the clutch assembly 1100 allows to the drive cable 116to be alternately operated between the drive mode and the release mode,as dictated by the magnitude of the applied torque T. To this end, theclutch assembly 1100 includes a driven member 1102 and a driver member1104, which are conditionally affixed to each other. That is, the drivenmember 1102 is rotatably engaged with the driver member 1104 before thecurrent magnitude of the applied torque T exceeds the criticalmagnitude, and is rotatably disengaged from the driver member 1104 afterthe current magnitude of the applied torque T exceeds the criticalmagnitude. Unlike the clutch assemblies 200, 700, and 1000, however, theclutch assembly 1100 utilizes rigid bodies to effect the frictionalclutching action.

Specifically, the driver member 1104 comprises a generally cylindricalrigid member 1114, which is molded from a suitably rigid material, e.g.,plastic. The cylindrical member 1114 includes a proximally facingreceptacle 1116 with a cavity 1118 formed therein for receiving thedistal end of a rigid motor drive shaft 184 from the MDU 104, when thecatheter hub 132 is mated with the MDU hub 186 (shown in FIG. 2). Tofacilitate proper and firm engagement with the motor drive shaft 184,the receptacle 1116 and motor drive shaft 184 are keyed, such that thereceptacle 1116 rotatably engages the motor drive shaft 184 wheninserted into the cavity 1118. The cylindrical member 1114 furtherincludes a distally facing receptacle 1120 with a cavity 1122 formedtherein.

The driven member 1102 comprises a generally cylindrical rigid member1108, which is composed of a suitable rigid material, e.g., stainlesssteel. The cylindrical member 1108 includes an elongate shaft 1110 witha proximally facing boss 1112. The boss 1112 and the shaft 1110 can bemolded as an integral unit, or can alternatively be to affixed to eachother using suitable means, e.g., welding.

The boss 1112 is disposed within the cavity 1122 of the receptacle 1120,with the outer diameter of the boss 1112 being slightly less than thediameter of the cavity 1122, such that, absent any external bindingforce, the boss 1112 can rotate freely within the cavity 1122. Tofacilitate axial alignment between the driven member 1102 and drivermember 1104, the proximal face of the boss 1112 includes a centered pin1124 (shown in phantom in FIG. 16), and the receptacle 1120 includes acentered pin hole 1126 (also shown in phantom) proximally adjacent thecavity 1122, wherein the pin 1124 and pin hole 1126 engage each other tocenter the boss 1112 within the cavity 1122 of the receptacle 1120.

The spring clamp 1106 is interference fit about the receptacle 1120 andboss 1112 to provide a binding force between the receptacle 1120 andboss 1112. Specifically, the longitudinal center of the receptacle 1120includes a pair of opposing circumferential cutouts 1128 and a pair ofadjacent bridge sections 1130. Thus, the boss 1112 includes a pair ofopposing arcuate surfaces 1132 that is exposed through the respectivecutouts 1128. The spring clamp 1106 is interference fit around the pairof bridge sections 1130 and the pair of exposed arcuate surfaces 1132,such that a frictionally engaging relationship is formed among thespring clamp 1106, receptacle 1120, and boss 1112.

Once the critical magnitude is exceeded, the current magnitude of theapplied torque T overcomes the frictional force generated by theinterference fit among the spring clamp 1106, receptacle 1120, and boss1112. Thus, the driven member 1102 becomes rotatably unengaged with thedriver member 1104. As a result, the drive cable 116 is rotatablyuncoupled from, and does not integrally rotate with, the motor driveshaft 184. That is, the drive cable 116 is operated in the release mode.

FIGS. 18 and 19 depict another alternative embodiment of an automaticclutch assembly 1200, which is constructed in accordance with thepresent inventions. Like the clutch assembly 1100 described above, theclutch assembly 1200 includes a driven member 1202 and a driver member1204 that are conditionally affixed to each other, wherein the clutchingfunction of the clutch assembly 1200 is frictionally actuated by thespring clamp 1106. The clutch assembly 1200 differs from the clutchassembly 1100, however, in that the driven member 1202 houses the drivermember 1204, rather than vice versa.

Specifically, the driven member 1202 includes a generally cylindricalrigid member 1208, which is constructed similarly to the above-describedcylindrical member 1108 (see FIG. 16), with the exception that thecylindrical member 1208 includes a proximally facing receptacle 1212having a cavity 1213 formed therein, rather than the boss 1112. Thedriver member 1204 includes a generally cylindrical rigid member 1214,which is constructed similarly to the above-described cylindrical member1114 (see FIG. 16), with the exception that the cylindrical member 1214includes a distally facing transitional shaft 1220, rather than thetransitional shaft 1120.

The transitional shaft 1220 is disposed within the cavity 1213 of thereceptacle 1212, with the outer diameter of the transitional shaft 1220being slightly less than the diameter of the cavity 1213, such that,absent any external binding force, the transitional shaft 1220 canrotate freely within the cavity 1213. To facilitate axial alignmentbetween the driven member 1202 and driver member 1204, the distal faceof the transitional shaft 1220 includes a centered pin 1224 (shown inphantom in FIG. 18), and the receptacle 1212 includes a centered pinhole 1226 (also shown in phantom in FIG. 18) distally adjacent thecavity 1213, wherein the pin 1224 and pin hole 1226 engage each other tocenter the transitional shaft 1220 within the cavity 1213 of thereceptacle 1212.

The spring clamp 1106 is interference fit about the receptacle 1212 andtransitional shaft 1220 to provide a binding force between thereceptacle 1212 and transitional shaft 1220. Specifically, thelongitudinal center of the receptacle 1212 includes a pair of opposingcircumferential cutouts 1228 and a pair of adjacent bridge sections1230. Thus, the transitional shaft 1220 includes a pair of opposingarcuate surfaces 1232 that is exposed through the respective cutouts1228. The spring clamp 1106 is interference fit around the pair ofbridge sections 1230 and the pair of exposed arcuate surfaces 1232, suchthat a frictionally engaging relationship is formed among the springclamp 1106, receptacle 1212, and transitional shaft 1220.

The operation of the clutch assembly 1200 is identical to that of theclutch assembly 1100, with the exception that the spring clamp 1106, thereceptacle 1212 of the driven member 1202, and transitional shaft 1220of the driver member 1204 frictionally interact with each other, ratherthan the spring clamp 1106, receptacle 1120 of the driver member 1104,and boss 1112 of the driven member 1102.

FIGS. 20-22 depict another alternative embodiment of an automatic clutchassembly 1300, which is constructed in accordance with the presentinventions. Like the clutch assemblies 200, 700, 1000, and 1100described above, the clutch assembly 1300 allows the drive cable 116 tobe alternately operated between the drive mode and the release mode, asdictated by the magnitude of the applied torque T. To this end, theclutch assembly 1300 includes a driven member 1302 and a driver member1304, which are conditionally affixed to each other. That is, the drivenmember 1302 is rotatably engaged with the driver member 1304 before thecurrent magnitude of the applied torque T exceeds the criticalmagnitude, and is rotatably disengaged from the driver member 1304 afterthe current magnitude of the applied torque T exceeds the criticalmagnitude. Unlike the clutch assemblies 200, 700, 1000, and 1100,however, the clutch assembly 1300 utilizes magnetic forces, rather thanfrictional forces, to effect the clutching action.

Specifically, the driver member 1304 comprises a generally cylindricalrigid member 1314, which is molded from a ferrous material. Thecylindrical member 1314 includes a proximally facing receptacle 1316with a cavity 1318 formed therein for receiving the distal end of arigid motor drive shaft 184 from the MDU 104, when the catheter hub 132is mated with the MDU hub 186 (shown in FIG. 2). To facilitate properand firm engagement with the motor drive shaft 184, the receptacle 1316and motor drive shaft 184 are keyed, such that the receptacle 1316rotatably engages the motor drive shaft 184 when inserted into thecavity 1318. The cylindrical member 1314 further includes a distallyfacing transitional shaft 1320.

The driven member 1302 comprises a generally cylindrical rigid member1308, which is composed of a suitable rigid material, e.g., stainlesssteel. The cylindrical member 1308 includes an elongate shaft 1310 witha proximally facing receptacle 1312 having a cavity 1313 formed therein.The receptacle 1312 and the shaft 1310 can be molded as an integralunit, or can alternatively be affixed to each other using suitablemeans, e.g., welding.

The transitional shaft 1320 is disposed within the cavity 1313 of thereceptacle 1312. To facilitate axial alignment between the driven member1302 and driver member 1304, the distal face of the transitional shaft1320 includes a centered pin 1324 (shown in phantom in FIG. 20), and thereceptacle 1312 includes a centered pin hole 1326 (also shown inphantom) distally adjacent the cavity 1313, wherein the pin 1324 and pinhole 1326 engage each other to center the transitional shaft 1320 withinthe cavity 1313 of the receptacle 1312.

A magnetic system provides the means for effecting the aforementionedclutching action. Specifically, the transitional shaft 1320 of thecylindrical member 1314 is composed of a ferrous material, and includesfour outwardly extending permanent magnets 1328, which arecircumferentially affixed about the transitional shaft 1320 by suitablemeans, e.g., bonding. In the illustrated embodiment, adjacent magnets1328 are separated by 90° and substantially extend the length of thetransitional shaft 1320. As can be seen, each magnet 1328 includes anorth pole N and a south pole S, with the polarities of each magnet 1328being opposite with respect to the two adjacent magnets 1328.

The receptacle 1312 of the cylindrical member 1308 is composed of aferrous material, and includes four inwardly extending ferrous elements1330 and four outwardly extending ferrous arcs 1332, which arecircumferentially disposed about the cavity 1313. In the illustratedembodiment, adjacent ferrous elements 1330 are separated by 90° andsubstantially extend the length of the receptacle 1312. The ferrous arcs1332 are interlaced between the ferrous elements 1330, and likewise, areseparated by 90° and substantially extend the length of the receptacle1312. In the embodiment illustrated in FIG. 21, the ferrous elements1330 and arcs 1332 are formed from the deformed inner surface of thereceptacle 1312. In an alternative embodiment illustrated in FIG. 22,the ferrous elements 1330 and arcs 1332 are formed from four curvilinearflanges.

The four ferrous elements 1330 are located outwardly adjacent the fourmagnets 1328, respectively, such that a magnetically engagingrelationship is formed between the magnets 1328 and ferrous elements1330. As can be seen, the transitional shaft 1320, by virtue of itsferrous composition, advantageously provides a-magnetic return(indicated by arrows) between the inward poles of adjacent magnets 1328,and the receptacle 1312, by virtue of its ferrous composition, providesa magnetic return (indicated by arrows) between the outward poles ofadjacent magnets 1328. The outwardly extending arcs 1332 facilitate themagnetically engaging relationship between the magnets 1328 and ferrouselements 1330, by concentrating the magnetic force at the ferrouselements 1330.

The operation of the clutch assembly 1300 is similar to that of theclutch assembly 200 described with respect to FIG. 5. Specifically, aslong as the critical magnitude is not exceeded, the current magnitude ofthe applied torque T does not overcome the attractive magnetic forcegenerated between the magnets 1328 and ferrous elements 1330. Thus, thedriven member 1302 remains rotatably engaged with the driver member1304. As a result, the drive cable 116 is rotatably coupled to, andintegrally rotates with, the motor drive shaft 184. That is, the drivecable 116 is operated in the drive mode.

Once the critical magnitude is exceeded, the current magnitude of theapplied torque T overcomes the attractive magnetic force generatedbetween the magnets 1328 and ferrous elements 1330. Thus, the drivenmember 1302 becomes rotatably unengaged with the driver member 1304. Asa result, the drive cable 116 is rotatably uncoupled from, and does notintegrally rotate with, the motor drive shaft 184. That is, the drivecable 116 is operated in the release mode.

FIGS. 23-25 depict another alternative embodiment of an automatic clutchassembly 1400, which is constructed in accordance with the presentinventions. Like the clutch assembly 1300 described above, the clutchassembly 1400 includes a driven member 1402 and a driver member 1404that are conditionally affixed to each other, wherein the clutchingfunction of the clutch assembly 1400 is magnetically actuated. Theclutch assembly 1400 differs from the clutch assembly 1300, however, inthat the driver member 1404 houses the driven member 1402, rather thanvice versa. Also, the driven member 1402 is magnetic and the drivermember 1404 is ferrous, rather than vice versa.

Specifically, the driven member 1402 includes a generally cylindricalrigid member 1408, which is constructed similarly to the above-describedcylindrical member 1308 (see FIG. 20), with the exception that thecylindrical member 1408 includes a proximally facing boss 1412, ratherthan the receptacle 1312. The driver member 1404 includes a generallycylindrical rigid member 1414, which is constructed similarly to theabove-described cylindrical member 1314 (see FIG. 20), with theexception that the cylindrical member 1414 includes a distally facingreceptacle 1420 with a cavity 1422 formed therein, rather than thetransitional shaft 1320.

The boss 1412 is disposed within the cavity 1422 of the receptacle 1420.To facilitate axial alignment between the driven member 1402 and drivermember 1404, the distal face of the boss 1412 includes a centered pin1424 (shown in phantom in FIG. 23), and the receptacle 1420 includes acentered pin hole 1426 (also shown in phantom in FIG. 23) proximallyadjacent the cavity 1422, wherein the pin 1424 and pin hole 1426 engageeach other to center the boss 1412 within the cavity 1422 of thereceptacle 1420. The boss 1412 is composed of a ferrous material, andincludes four outwardly extending permanent magnets 1428, which arecircumferentially affixed about the boss 1412 by suitable means, e.g.,bonding.

The receptacle 1420 is composed of a ferrous material, and includes fourinwardly extending ferrous elements 1430 and four outwardly extendingferrous arcs 1432, which are circumferentially disposed about the cavity1422. In the embodiment illustrated in FIG. 24, the ferrous elements1430 and arcs 1432 are formed from the deformed inner surface of thereceptacle 1420. In an alternative embodiment illustrated in FIG. 25,the ferrous elements 1430 and arcs 1432 are formed from four curvilinearflanges.

The four ferrous elements 1430 are located outwardly adjacent the fourmagnets 1428, respectively, such that a magnetically engagingrelationship is formed between the magnets 1428 and ferrous elements1430. As can be seen, the boss 1412, by virtue of its ferrouscomposition, advantageously provides a magnetic return (indicated byarrows) between the inward poles of adjacent magnets 1428, and thereceptacle 1420, by virtue of its ferrous composition, provides amagnetic return (indicated by arrows) between the outward poles ofadjacent magnets 1428. The outwardly extending arcs 1432 facilitate themagnetically engaging relationship between the magnets 1428 and ferrouselements 1430, by concentrating the magnetic force at the ferrouselements 1430.

The operation of the clutch assembly 1400 is identical to that of theclutch assembly 1300, with the exception that the magnets 1428 of thedriven member 1402 and the ferrous elements 1430 of the driver member1404 magnetically interact with each other, rather than the magnets 1328of the driver member 1304 and the ferrous elements 1330 of the drivenmember 1302.

FIGS. 26-28 depict another alternative embodiment of an automatic clutchassembly 1500, which is constructed in accordance with the presentinventions. Like the clutch assembly 1300 described above, the clutchassembly 1500 includes a driven member 1502 and a driver member 1504that are conditionally affixed to each other, wherein the clutchingfunction of the clutch assembly 1500 is magnetically actuated. Theclutch assembly 1500 differs from the clutch assembly 1300, however, inthat the driven member 1502 is magnetic and the driver member 1504 isferrous, rather than vice versa.

Specifically, the driven member 1502 includes a generally cylindricalrigid member 1508, which is constructed similarly to the above-describedcylindrical member 1308 (see FIG. 20), and includes a proximally facingreceptacle 1512 having a cavity 1513 formed therein. The driver member1504 includes a generally cylindrical rigid member 1514, which isconstructed similarly to the above-described cylindrical member 1314(see FIG; 20), and includes a transitional shaft 1520.

The transitional shaft 1520 is disposed within the cavity 1513 of thereceptacle 1512. The receptacle 1512 is composed of a ferrous material,and includes four inwardly extending permanent magnets 1528, which arecircumferentially disposed around the cavity 1513, and are affixed tothe receptacle 1512 by suitable means, e.g., bonding. The transitionalshaft 1520 is composed of a ferrous material, and includes fouroutwardly extending ferrous elements 1530 and four inwardly extendingferrous arcs 1532, which are circumferentially disposed around thetransitional shaft 1520. In the embodiment illustrated in FIG. 27, theferrous elements 1530 and arcs 1532 are formed from the deformed outersurface of the transitional shaft 1520. In an alternative embodimentillustrated in FIG. 28, the ferrous elements 1530 and arcs 1532 areformed from four curvilinear flanges.

The four ferrous elements 1530 are located inwardly adjacent the fourmagnets 1528, respectively, such that a magnetically engagingrelationship is formed between the magnets 1528 and ferrous elements1530. As can be seen, the receptacle 1512, by virtue of its ferrouscomposition, advantageously provides a magnetic return (indicated byarrows) between the outward poles of adjacent magnets 1528, and thetransitional shaft 1520, by virtue of its ferrous composition, providesa magnetic return (indicated by arrows) between the inward poles ofadjacent magnets 1528. The inwardly extending arcs 1532 facilitate themagnetically engaging relationship between the magnets 1528 and ferrouselements 1530, by concentrating the magnetic force at the ferrouselements 1530.

The operation of the clutch assembly 1500 is identical to that of theclutch assembly 1300, with the exception that the magnets 1528 of thedriven member 1502 and the ferrous elements 1530 of the driver member1504 magnetically interact with each other, rather than the magnets 1328of the driver member 1304 and the ferrous elements 1330 of the drivenmember 1302.

FIGS. 29-31 depict another alternative embodiment of an automatic clutchassembly 1600, which is constructed in accordance with the presentinventions. Like the clutch assembly 1300 described above, the clutchassembly 1600 includes a driven member 1602 and a driver member 1604that are conditionally affixed to each other, wherein the clutchingfunction of the clutch assembly 1600 is magnetically actuated. Theclutch assembly 1600 differs from the clutch assembly 1300, however, inthat the driver member 1604 houses the driven member 1602, rather thanvice versa.

Specifically, the driven member 1602 includes a generally cylindricalrigid member 1608, which is constructed similarly to the above-describedcylindrical member 1308 (see FIG. 20), with the exception that thecylindrical member 1608 includes a proximally facing boss 1612, ratherthan a receptacle 1312. The driver member 1604 includes a generallycylindrical rigid member 1614, which is constructed similarly to theabove-described cylindrical member 1314 (see FIG. 20), with theexception that the cylindrical member 1614 includes a distally facingreceptacle 1620 having a cavity 1622 formed therein, rather than atransitional shaft 1320.

The boss 1612 is disposed within the cavity 1622 of the receptacle 1620.The receptacle 1620 is composed of a ferrous material, and includes fourinwardly extending permanent magnets 1628, which are circumferentiallydisposed around the cavity 1622, and are affixed to the receptacle 1620by suitable means, e.g., bonding. The boss 1612 is composed of a ferrousmaterial, and includes four outwardly extending ferrous elements 1630and four inwardly extending ferrous arcs 1632, which arecircumferentially disposed around the boss 1612. In the embodimentillustrated in FIG. 30, the ferrous elements 1630 and arcs 1632 areformed from the deformed outer surface of the boss 1612. In analternative embodiment illustrated in FIG. 31, the ferrous elements 1630and arcs 1632 are formed from four curvilinear flanges.

The four ferrous elements 1630 are located inwardly adjacent the fourmagnets 1628, respectively, such that a magnetically engagingrelationship is formed between the magnets 1628 and ferrous elements1630. As can be seen, the receptacle 1620, by virtue of its ferrouscomposition, advantageously provides a magnetic return (indicated byarrows) between the outward poles of adjacent magnets 1628, and the boss1612, by virtue of its ferrous composition, provides a magnetic return(indicated by arrows) between the inward poles of adjacent magnets 1628.The inwardly extending arcs 1632 facilitate the magnetically engagingrelationship between the magnets 1628 and ferrous elements 1630, byconcentrating the magnetic force at the ferrous elements 1630.

The operation of the clutch assembly 1600 is identical to that of theclutch assembly 1300, with the exception that the magnets 1628 of thedriven member 1602 and the ferrous elements 1630 of the driver member1604 magnetically interact with each other, rather than the magnets 1328of the driver member 1304 and the ferrous elements 1330 of the drivenmember 1302.

With regard to any of the above-described clutch assemblies, thecritical magnitude of the applied torque T, i.e., the point at which thedriven member and driver member are rotatably uncoupled from each other,can be selected by “tuning” these clutch assemblies, i.e., altering thematerials from which the elements are composed, altering the size of orspatial relationship between the elements, etc. To ensure properclutching action, a simple fixture with a built-in torque watch can beused to apply a measured torque to these clutch assemblies, whereby thecritical magnitude of the applied torque can be determined and comparedagainst an optimum critical magnitude.

While preferred embodiments have been shown and described, it will beapparent to one of ordinary skill in the art that numerous alterationsmay be made without departing from the spirit or scope of the invention.Therefore, the invention is not to be limited except in accordance withthe following claims.

What is claimed:
 1. A catheter, comprising: an elongate member; arotatable catheter drive shaft disposed within the elongate member, thecatheter drive shaft having a proximal end to which torque can beapplied; an imaging device mounted on the distal portion of therotatable catheter drive shaft: and a clutch assembly mechanicallycoupled to the proximal end of the catheter drive shaft, and configuredsuch that the catheter drive shaft operates in a drive mode before theapplied torque exceeds a critical magnitude, and operates in a releasemode after the applied torque exceeds the critical magnitude.
 2. Thecatheter of claim 1, further comprising a proximal catheter hub mountedto the elongate member, wherein the proximal catheter hub houses theclutch assembly.
 3. The catheter of claim 1, wherein the elongate membercomprises a telescoping guide sheath slidably disposed about thecatheter drive shaft.
 4. The catheter of claim 1, wherein the clutchassembly is frictionally actuated.
 5. The catheter of claim 1, whereinthe clutch assembly is magnetically actuated.
 6. The catheter of claim1, further comprising an operative element mounted on a distal end ofthe catheter drive shaft.
 7. The catheter of claim 1, wherein theimaging device includes an ultrasonic transducer and the catheter driveshaft includes at least one wire adapted to communicate imaging signalsfrom the ultrasonic transducer. an elongate member; a rotatable catheterdrive shaft to which torque can be applied, the catheter drive shaftbeing disposed within the elongate member; a driven member rotatablycoupled to a proximal end of the catheter drive shaft; and a drivingmember cooperating with the driven member, wherein the driven anddriving members are rotatably engaged with each other before the appliedtorque exceeds a critical magnitude, and rotatably unengaged with eachother after the applied torque exceeds the critical magnitude.
 8. Thecatheter of claim 1, further comprising an operative element mounted ona distal end of the catheter drive shaft.
 9. The catheter of claim 8,wherein the catheter drive shaft and operative element form anultrasonic imaging core.
 10. A catheter, comprising: an elongate member;a rotatable catheter drive shaft to which torque can be applied, thecatheter drive shaft being disposed within the elongate member; a drivenmember rotatably coupled to a proximal end of the catheter drive shaft;and a driving member cooperating with the driven member, wherein thedriven and driving members are rotatably engaged with each other beforethe applied torque exceeds a critical magnitude, and rotatably unengagedwith each other after the applied torque exceeds the critical magnitude.11. The catheter of claim 10, further comprising a proximal catheter hubmounted to the elongate member, wherein the proximal catheter hub housesthe driven member and driving member.
 12. The catheter of claim 10,wherein the elongate member comprises a telescoping guide sheathslidably disposed about the catheter drive shaft.
 13. The catheter ofclaim 10, wherein the driven and driving members are rotatably engagedby frictional forces.
 14. The catheter of claim 10, wherein the drivenand driving members are rotatably engaged by magnetic forces.
 15. Acatheter comprising: an elongate catheter body; a catheter drive shaftdisposed within the elongate catheter body; a motor adapted to rotatethe catheter drive shaft at a torque; and a clutch mechanically coupledto the motor and to the catheter drive shaft, the clutch allowing themotor to rotate the catheter drive shaft before the torque exceeds athreshold and automatically preventing the motor from rotating thecatheter drive shaft when the torque exceeds the threshold, the clutchhaving hysteresis allowing the motor to resume rotation of the catheterdrive shaft only when the torque falls substantially below thethreshold.
 16. The catheter of claim 15, further comprising a proximalcatheter hub mounted to the elongate catheter body, wherein the proximalcatheter hub houses the clutch.
 17. The catheter of claim 15, whereinthe elongate catheter body comprises a telescoping guide sheath slidablydisposed about the catheter drive shaft.
 18. The catheter of claim 15,wherein the clutch is frictionally actuated.
 19. The catheter of claim15, wherein the clutch is magnetically actuated.
 20. The catheter ofclaim 15, further comprising an imaging device mounted on a distal endof the catheter drive shaft.
 21. The catheter of claim 20, wherein theimaging device comprises an ultrasound imaging device.
 22. The catheterof claim 15, wherein the clutch comprises a driven member and a drivingmember adapted to cooperate with the driven member.
 23. The catheter ofclaim 22, wherein the driven and driving members are rotatably engagedfrictionally.
 24. The catheter of claim 22, wherein the driven anddriving members are rotatably engaged magnetically.
 25. The catheter ofclaim 15, further comprising a treatment device mounted on a distal endof the catheter drive shaft.
 26. The catheter of claim 25, furthercomprising an imaging device mounted on a distal end of the catheterdrive shaft.
 27. The catheter of claim 15, wherein the motor drives thecatheter drive shaft at a substantially uniform speed.
 28. A cathetercomprising: an elongate member; a catheter drive shaft disposed withinthe elongate member; a motor adapted to rotate and apply torque to thecatheter drive shaft; an automated clutch assembly coupled between thecatheter drive shaft and the motor for alternately operating thecatheter drive shaft in a drive mode and a release mode, where thecatheter drive shaft and the motor are rotatably engaged with each otherbefore the applied torque exceeds a threshold, the catheter drive shaftand the motor are automatically disengaged from each other when theapplied torque exceeds the threshold, and hysteresis in the clutchassembly prevents the catheter drive shaft and the motor fromre-engaging each other until the applied torque falls substantiallybelow the threshold.
 29. The catheter of claim 28 wherein the motor hasa motor drive shaft that engages the catheter drive shaft.
 30. Thecatheter of claim 28 wherein hysteresis in the clutch assembly preventsthe catheter drive shaft and the motor from re-engaging each other untilthe applied torque falls below a second threshold.
 31. The catheter ofclaim 28, further comprising a proximal catheter hub mounted to theelongate element, wherein the proximal catheter hub houses the clutchassembly.
 32. The catheter of claim 28, wherein the elongate elementcomprises a telescoping guide sheath slidably disposed about thecatheter drive shaft.
 33. The catheter of claim 28, wherein the clutchassembly is frictionally actuated.
 34. The catheter of claim 28, whereinthe clutch assembly is magnetically actuated.
 35. The catheter of claim28, further comprising an imaging device mounted on a distal end of thecatheter drive shaft.
 36. The catheter of claim 35, wherein the imagingdevice comprises an ultrasound imaging device.
 37. The catheter of claim28, wherein the clutch comprises a driven member and a driving memberadapted to cooperate with the driven member.
 38. The catheter of claim37, wherein the driven and driving members are rotatably engagedfrictionally.
 39. The catheter of claim 37, wherein the driven anddriving members are rotatably engaged magnetically.
 40. The catheter ofclaim 28, further comprising a treatment device mounted on a distal endof the catheter drive shaft.
 41. The catheter of claim 40, furthercomprising an imaging device mounted on a distal end of the catheterdrive shaft.
 42. The catheter of claim 28, wherein the motor drives thecatheter drive shaft at a substantially uniform speed.